Two experiments with obsessive-compulsive patients are proposed to test the efficacy of modalities of treatment by exposure and response prevention. The first study examines the differential effectiveness of exposure alone or response prevention alone or a combination of the two. Exposure and response prevention combined are highly stressful for patients. If either component alone yields similar results, it will constitute a less stressful treatment alternative and consequently a preferable one. Additionally, this study will provide information about theoretical issues basic to the exposure paradigm. The second study will test the effectiveness of exposure in imagination to ruminations of anticipated disasters in patients who are checkers or washers. It is proposed to study here the differential effect of exposure in imagination or exposure in vivo. This study bears on the general issue of the relationship between cognitive and behavioral events. Early symptoms of failure to respond to behavioral treatment also will be explored. In a pilot study all three components, exposure in vivo, exposure in imagination and response prevention, were employed simultaneously. The proposed studies will examine the differential effects of each component treatment to devise optimal programs for different groups of obsessive-compulsives.